1. Field of the Invention
The present invention relates to: a photodetection device which is a device having a photoelectric conversion function and configured to detect light of wavelengths in a specific range; and an optical filter used in the photodetection device.
2. Description of the Related Art
Some photodetection devices use what is called a photoconductive sensor element configured to detect ultraviolet light applied onto a light receiving portion thereof on the basis of change in the amount of photoinduced current in the light receiving portion. Si semiconductor and the like having detection sensitivity to visible light in a wavelength range from 400 nm to 750 nm and the like have conventionally been considered for this photoconductive sensor element, for its inexpensiveness and easiness in controlling doping. The principle of photodetection of the photoconductive sensor element is as follows. As the semiconductor of the light receiving portion is irradiated with light having energy equal to or greater than the bandgap, electron-hole pairs are created in the semiconductor by a photoelectric conversion function, and these carriers are taken out to an external circuit by an externally applied voltage and detected as an amount of photoinduced current.
Conventional photoelectric conversion elements are generally made of Si as mentioned above. However, since Si is sensitive to all the wavelength ranges shorter than 1.1 μm, it is impossible to take out only light of specific wavelengths and measure the amount of that light.
In this respect, a visible light cut filter is generally an interference filter in which films with mutually different normal refractive indexes are alternately stacked. However, the cut bandwidth is determined by the difference between the refractive indexes of the films to be used, and thus, it is difficult to set the entire visible light range from 400 to 800 nm as the wavelength range within which the interference filter can cut the light amount down to approximately 0. In addition, the interference filter inevitably has uncuttable wavelengths, so that the interference filter may be able to cut visible light but fails to cut infrared light. In this case, it is difficult to transmit and measure only ultraviolet light because the Si photoelectric conversion element is sensitive also to infrared light.
Meanwhile, for the purpose of solving the above problem that occurs due to the combination of an interference filter and an Si photoelectric conversion element, a configuration has been proposed in which light is detected while setting mutually different light receiving sensitivity wavelength ranges by making the depths of pn junction interfaces, i.e., the depths of photoelectric conversion regions, different from each other. Specifically, one pn junction interface is formed shallow so that light can be detected by the photoelectric conversion region having excellent sensitivity characteristics to relatively short wavelength ranges. The other pn junction interface is formed deep so that light can be detected by the photoelectric conversion region having a fine sensitivity to long wavelengths. Then, the difference between the two detected signals is calculated. As a result, short wavelength light can be detected (see Japanese Patent Application Publication Nos. 2009-158570, 2007-67331, 2002-164565, 2009-158928, 2007-305868, 2006-318947, for example).
In this case, however, the sensitivity to the ultraviolet range is poor. In addition, the depths of the pn junctions need to be adjusted for each detection target wavelength range in the ultraviolet range, which is extremely troublesome. Moreover, it is difficult to detect only light in the ultraviolet range regardless of how the depths of the pn junctions are adjusted.
Meanwhile, there is a configuration as described in Patent Document 2 in which: the same depth is set for the pn junctions of two photoelectric conversion regions; an ultraviolet light absorption film configured to absorb part of ultraviolet light is formed on one of photodiodes; and the difference between the photodiodes is figured out. However, the ultraviolet light absorption film is considered as a film configured to absorb part of ultraviolet light, and as described in the document, is a film that can only weaken ultraviolet light to be received, by absorbing part of the ultraviolet light. Since the level of the ultraviolet light absorption is weak, the detection sensitivity obtained from the calculation of the difference is weak as well.
Further, the following problem occurs when the difference between a photodiode A with an optical filter formed thereon and a photodiode B provided with no optical filter is figured out as in Patent Document 2. In the photodiode A with the optical filter formed thereon, interference fringes are generated as a result of the interference between reflected light from the surface of the optical filter and reflected light from the interface between the optical filter and the semiconductor layer. Consequently, the photodetection signal comes to contain a signal caused by the interference fringes, making accurate detection impossible.
As described above, it has been difficult to detect measurement-target light in a specific wavelength range selectively with a high sensitivity.